This invention relates to a power servo of the fluid differential pressure actuated type, and more particularly to a device for escaping air confined in a space defined between a diaphragm and a power piston to promote return speed of the piston during the non-servo period. The specific structural implementation, embodying the invention aids in minimizing working period for discharging air in brake circuit.
A conventional power servo booster is shown in FIG. 1, wherein a power piston is connected to a control rod 8 in turn coupled to a vehicle operator controlled brake pedal (not shown). The power piston 1 comprises a disc portion 1a, a flange portion 1c, an annular groove 1b formed between the disc 1a and the flange 1c, and a sleeve portion 1d extending from the peripheral end of the disc portion 1a.
A diaphragm 2 is secured to the piston 1 and a housing consisting of front and rear shells 3 and 4, and cooperates with the piston and the housing to divide the housing into differential pressure chambers, namely an atmospheric chamber 5 and a negative chamber 6. The inner peripheral end of the diaphragm is secured to the annular groove 1b and the outer peripheral end thereof is secured to the front and rear shells. A radially outward extending section 2a of the diaphragm 2 is in surface engagement with the disc portion 1a of the piston 1, and a folded back portion 2b of the diaphragm 2 is in surface engagement of the sleeve 1d under pressure. Reference numeral 7 designates a negative pressure passage connected to an intake manifold to provide negative pressure in the chamber 6. Servos of this type are shown in U.S. Pat. Nos. 3,136,229 and 3,183,789.
With this Prior Art structure, during normal operation, in which negative pressure is introduced in the chamber 6 gas communication between the negative chamber 6 and the atmospheric chamber 5 is blocked by conventional valve means (not shown). When the control rod 8 is pushed towards the left, simultaneously, air is introduced in the atmospheric chamber 5 to provide a pressure differential therebetween to thereby provide urging force of the power piston 1. A push rod 9 connected to a master cylinder (not shown) is moved requiring only a small pressing force of a brake-foot pedal.
Upon releasing the control rod 8, the atmospheric chamber 5 communicates with the negative chamber 6 to provide equal pressure therein, so that the power piston 1 together with the diaphragm 2 are restored to their original position by a biasing force of a spring 10.
Generally, the radially outward extending section 2a and the fold back portion 2b of the diaphragm 2 are in surface engagement with the disc portion 1a and the sleeve portion 1d of the power piston 1, respectively, during the above described power-servo operation. However, when negative pressure is not introduced from the passage 7, such as during air discharging work in the brake piping at the time of the safety checkup of the product, or during assembly, the radially outward extending section 2a may be spaced apart from the disc 1a urging the control rod 8 inward due to no pressure differential between the chambers 5 and 6. The surface engagement between the sleeve 1d and the folded back portion 2b is released.
Therefore air may accumulate in a space defined between the extending section 2a and the disc 1a as shown in FIG. 2 during the return stroke of the power piston. The air accumulated in the space prevents the piston from moving rearwardly, so that return speed of the piston is seriously decreased. That is, since it is necessary to compress the confined air, to return the piston and the force for compressing the confined air opposes the rearward movement of the piston, due to Boyle's law. This trapped air then inpedes operation of the servo.
In the prior art a recognition of this problem exists as shown in Japanese Utility Model No. 1166698 dated Mar. 31, 1977. Specifically, this prior shows in FIG. 2 that under some conditions, for example when the piston is rest after non-powered operation, an air chamber may be created between the disc and the diaphragm. The prior art overcomes the problem by placement of a port [18 in FIG. 1 of the Utility Model] so that opposite surfaces of the disc communicate with the constant pressure chamber (negative side). The Utility Model specifically introduces pressure from the constant pressure chamber through a port formed in the piston to separate the piston and the diaphragm except for a small portion of the diaphragm located near the inner periphery. This reduces the effective diameter of the diaphragm during compression to reduce pedaling force. As a result, during non-powered operation of the brake booster, the pressure differential produced between the constant pressure section and the variable pressure section is reduced to make the loss of brake pedaling pressure very small.
During reset, when the constant pressure chamber is held at a reduced pressure, the port in the piston allows air to be removed from the space between the piston and the diaphragm thereby eliminating any air pocket that may tend to form. The Utility Model also discloses the use of axial slots formed in either the disc or the diaphragm is communication with the constant pressure chamber to expel any air that may be trapped during reseting of the servo device.
The system disclosed in the prior art Utility Model, while effective to eliminate air pockets introduces other problems into the system. By having ports disposed in the flange that element is structurally weakened. Also, the port must be of sufficient size to ensure that no local or small air pockets will be formed. This in turn causes a portion of the flange to be a discontenuous surface tending to deform the diaphragm. In extreme cases where the diaphragm may be weakened it may be extruded, in port, through such ports as a result of the pressure differential across the diaphragm. This may lead to eventual failure of the diaphragm with serious results.